1. Field of the Invention
The present invention relates to an electric bush cutter, and more particularly, to an electric bush cutter which adjusts the rotational speed of a cutting edge on the basis of an amount of operation of operating means for adjusting the rotational speed of an electric motor, and which can adjust the rotational speed of the cutting edge in a common-use region with high precision.
2. Description of the Related Art
In recent years, an electric bush cutter that is driven by an electric motor is drawing attention due to vibration and noise problems of a related bush cutter that is driven by an engine (refer to, for example, Japanese Unexamined Patent Application Publication No. 2006-217843). The electric bush cutter not only can overcome the vibration and noise problems, but also can be made lighter and can be easily handled during a bush-cutting operation.
Related electric bush cutters have various structures so that they can adjust the rotational speed of a cutting edge in accordance with the environment of a bush-cutting operation. The simplest structure allows, for example, two-step adjustments in an eco mode and a high-speed mode. In the eco mode, the cutting edge is rotated at a low speed to perform a bush-cutting operation. In addition, in the eco mode, when an electric motor is driven using a battery, consumption current of the battery is made as low as possible, to increase the operating time of the battery. That is, the eco mode is used when an ordinary number of plants is to be subjected to a bush-cutting operation, so that the working time when the battery is charged once is at least a certain time.
The high-speed mode is used when a load is applied to the cutting edge such as, when a rather thick, small branch is subjected to a bush-cutting operation. Since the cutting edge is rotated at a high speed, the consumption current of the battery is increased. That is, the high-speed mode is used under a particular condition, such as when the bush-cutting operation cannot be performed at the normal rotational speed of the cutting edge. It is assumed that, in the high-speed mode, the working time is not long.
Although a related bush cutter having a structure that allows the rotational speed of a cutting edge to change in two or more steps may be provided, the basic technical concept of such a structure is similar to that described above. That is, switching the rotational speed of the cutting edge to a predetermined rotational speed in steps in accordance with the load applied to the cutting edge keeps consumption of the battery power low, so that the bush-cutting time is lengthened.
The aforementioned switching of the rotational speed of the electric motor in steps can be arbitrarily performed by, for example, using a change-over switch to change in steps, for example, a pulse or a voltage applied to the electric motor.
A structure that can continuously change the rotational speed of a cutting edge is achieved by continuously changing an amount of operation of operating means for adjusting the rotational speed of an electric motor. For the operating means for adjusting the rotational speed of the electric motor, for example, a throttle lever can be used. Due to the amount of operation, that is, the opening degree of the throttle lever, the rotational speed of the cutting edge can be set in accordance with a bush-cutting environment. According to this structure, the rotational speed of the cutting edge can be adjusted when necessary by operating the throttle lever. Therefore, the consumption current of the battery is restricted, thereby making it possible to increase the overall working time.
In this case, it is desirable that the rotational speed of the cutting edge, that is, the rotational speed of the electric motor with respect to the amount of operation of the throttle lever changes substantially linearly. For example, Japanese Unexamined Patent Application Publication No. 5-122979 discloses a rotational-speed controlling circuit that can approximate the relationship between an amount of manual operation and the rotational speed of the motor to a linear relationship, even if lagging of supply current resulting from self-inductance of a motor coil occurs.
In the related methods of switching in steps the rotational speed of the cutting edge, it is possible to quickly perform the bush-cutting operation after a switch of the electric motor is switched on. However, it is not possible to adjust the rotational speed of the cutting edge when the bush-cutting operation is being performed. Ordinarily, the rotational speed of the motor suitable for the bush-cutting operation has a certain range, so that, if the rotational speed of the cutting edge can be adjusted to within this range, the bush-cutting operation can be efficiently carried out. For example, the range (common-use region) of the rotational speed of the cutting edge suitable for an ordinary bush-cutting operation may be set from 6000 rpm to 8000 rpm. In this case, the bush-cutting operation is performed at substantially 6000 rpm when the number of plants to be subjected to the bush-cutting operation is small, and, as the number of plants to be subjected to the bush-cutting operation is increased, the rotational speed is increased, so that, even when the number of plants to be subjected to the bush-cutting operation is a maximum, the bush-cutting operation is performed at substantially 8000 rpm. This increases the working efficiency and the operating time of the battery. However, in the methods of switching the rotational speed of the cutting edge in steps, the rotational speed of the cutting edge cannot be adjusted in accordance with the aforementioned conditions, thereby reducing the working efficiency. In addition, when the electric motor is driven by a battery, the amount of time that the battery can be used when the battery is charged once is reduced because the rotational speed of the cutting edge is unnecessarily increased.
In the method in which the rotational speed of the electric motor is substantially linearly changed by the amount of operation of, for example, a throttle lever, the throttle lever must be operated until the rotational speed of the electric motor in the common-use region is reached after the switch of the electric motor is switched on. The smaller the amount of operation of the throttle lever, the more quickly a bush-cutting operation can be started. However, since the rate of change of the rotational speed of the electric motor with respect to this operation is proportionally changed without changing this rate over the entire rotational speed region, a predetermined time is required until the rotational speed of the electric motor reaches the common-use region. Therefore, when the bush cutter is intermittently used, each starting time makes the bush cutter less easier to handle.
Further, when the rotational speed of the electric motor reaches the lowest limit of the rotational speed in the common-use region, the rotational speed of the cutting edge can be proportionally adjusted between the lowest limit of the rotational speed and the highest limit of the rotational speed in the common-use region. Therefore, the rotational speed of the cutting edge can be adjusted in accordance with the number of plants to be subjected to the bush-cutting operation. However, since the rate of change of the rotational speed with respect to the operation is set so as to be a constant value, that is, so as not to change over the entire rotational speed region, the rate of change of the rotational speed of the cutting edge with respect to the amount of operation is large. Therefore, it cannot be said that the bush cutter is suitable for the common-use region. That is, since the rotational speed of the electric motor with respect to a slight operation of, for example, the throttle lever changes by a large amount, the rotational speed cannot be adjusted with precision, thereby causing working stress.